Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Researchers have long postulated that living organisms utilize the macromolecules of organic matrices to actively guide the formation of crystal structures. The timing and placement of the subsequent minerals that form are most easily controlled during nucleation; however, a physical and chemical picture of how organic functional group chemistry influences the initial stages of nucleation is not yet established. These processes are further complicated by the realization that carbonate biominerals can form by an amorphous to crystalline transformation process, which has prompted the question of how chemical signatures are recorded during mineralization. Investigations of mineralization processes such as the kinetics of nucleation and the transformation of amorphous calcium carbonate (ACC) to crystalline products are critical to building a better understanding of biomineral formation. Only from that fundamental basis can one begin to decipher changes in climate and seawater chemistry over geologic time and by recent anthropogenic effects. This dissertation presents the findings from experimental studies of the thermodynamics and kinetics of multiple mineral formation processes, including nucleation and transformation from an amorphous phase. The kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates were quantified under controlled conditions. Nucleation rates were measured as a function of 1) supersaturation extending above and below ACC solubility and 2) ionic strength extending to seawater salinity. These conditions decipher the chemical interactions between the PS substrate, calcite crystal, and solution. These investigations show the energy barrier to calcite formation is regulated by competing interfacial energies between the substrate, crystal, and liquid. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. The data also suggest ionic strength regulates nucleation barriers through substrate-liquid and crystal-liquid interfacial energetics. In a second experimental study, stable isotope labeling was used to directly probe the transformation pathway. Four processes were considered: dissolution-reprecipitation, solid-state, or combinations of these end member processes. Isotope measurements of calcite crystals that transform from ACC have signatures that are best explained by dissolution-reprecipitation. The extent of isotopic mixing correlates with the amount of ACC transferred and the time to transformation, suggesting the calcite crystals are recording the changing local solution environment during the transformation. These investigations into different mineralization mechanisms build a framework for how functional group chemistries of organic molecules regulate mineralization and the resulting isotopic and elemental signatures in the calcite. This may provide useful insights to interpreting chemical signatures of carbonate biominerals in fossil record and understanding ocean chemistry changes throughout geologic time. / Ph. D.

The late Archean Mount McRae Shale of the Hamersley Basin in Western Australia may record the presence of oxygen in the atmosphere before the Great Oxidation Event (2.4-2.3 Ga). Several prior studies (Anbar et al., 2007; Blum and Anbar, 2010; Duan et al., 2010; Kakegawa et al., 1998; McManus et al., 2006) have used isotopic systems to analyze the Mount McRae Shale and conclude that there was a presence of oxygen before the Great Oxidation Event. The purpose of this study is to determine if the U-Pb system can be used to see through later events to the initial conditions. The uranium-lead values of the Mt McRae Shale provide evidence of the mobilization of U and Pb gain. The geochemical disturbances have been linked to the tectonic activity (460 Ma) in the neighboring Canning basin, which could have possibly opened the geochemical system. In terms of the depositional environment the U-Pb data gathered here do not point to oxygenation of the atmosphere.

The distributions of calcite rich horizons within dryland soils are commonly used as paleoclimate proxies. Comprehensive conceptual and mathematical models of calcite accumulation in soils are required to accurately interpret and calibrate these proxies. A conceptual model for calcite accumulation is already well established: As water percolates through a soil, it dissolves minerals, such as calcite, transporting the soluble minerals downward. As soil water is removed by evaporation and transpiration, the water solution becomes supersaturated resulting in precipitation of calcite at depth. The impacts of dynamic plant growth and microbial respiration have not yet been simulated in numerical models for calcite accumulation but are likely important because of their influence on variables governing calcite solubility. The soil hydraulic modeling software, HYDRUS-1D, simulates water and solute transfer through a soil column, accounting for variations in all previously studied variables (temperature, water content, soil pCO₂) while additionally simulating vegetation-soil interactions. Five separate sensitivity studies were conducted to determine the importance for calcite accumulation of 1) soil texture, 2) plant growth, 3) plant phenology, 4) atmospheric CO₂ concentrations, and 5) the proximal variables that control calcite dissolution and precipitation: soil CO₂, soil water content, and soil temperature. In each modeling simulation, calcite was leached from the top several cm and redistributed deeper in the soil after 20 years. Soils with courser texture yield deeper (+20cm), more diffuse calcite horizons, as did simulations with bare soil compared to vegetated soil. The phenology of plant communities (late spring versus late summer growth) resulted in soil calcite accumulation at temperatures differing by at least 10°C. Changes in atmospheric CO₂ concentrations do not affect the soil calcite distribution. Variations in concentration of soil CO₂, rather than soil water content, have the greatest direct effect on calcite solubility. The most significant time periods of annual accumulation also corresponded with positive water fluxes resulting from high matric potential at the surface. Transpiration and evaporation moisture sinks caused solution to travel upward from higher to lower soil CO₂ concentrations, causing CO₂ de-gassing and calcite accumulation. This pathway describes a new qualitative mechanism for soil calcite formation and should be included in the conceptual model. / text

The temperature record from the Antarctic Peninsula (AP) shows a warming trend 3°C greater than that of the Antarctic continent (Vaughan, et al., 2003). The LARsen Ice Shelf System, Antarctica (LARISSA) project was developed as an interdisciplinary collaboration to understand the impacts of global climate change on the ice shelf systems of the Peninsula. The 2010 LARISSA cruise to the western AP margin collected the two marine sediment cores from the mouth of Barilari Bay used for this thesis, Jumbo Piston Core (JPC) 127 and Jumbo Kasten Core (JKC) 55. The 77 sediment samples collected at 10 cm intervals were sieved at 63 microns to retain foraminiferal tests, identified to the species level. The 35 most abundant foraminifera species were grouped into five assemblages with one outlier species through Hierarchical Cluster Analysis (HCA), predominantly grouped by calcareous and agglutinated foraminifera. The Principal Component Analysis (PCA) yielded two principal components, which accounted for 81.5% of the variability within the data, correlated to the species Fursenkoina spp. and Bulimina aculeata. The base of this core was found to be nearly 8000 calibrated years before present (cal. yr. BP) through radiocarbon dating of the foraminiferal tests. The PCA results were correlated with the magnetic susceptibility down core, producing a timeline of four distinct zones in the mid- to late Holocene at the outer Barilari Bay core site. The earliest zone indicated stable cold bay waters, followed by a drastic change with the incursion of warmer Circumpolar Deep Water (CDW) onto the continental shelf. The third zone of this study illustrated a period of fluctuation between the cold bay waters and the CDW, interrupted by the Little Ice Age when the ice shelf in Barilari Bay extended to the mouth of the bay. The most recent zone depicts the past 200 years of melting ice shelves and the resulting increase in primary productivity observed in the bays of the western AP, discernable from the diatom, foraminifera, and sedimentological record. This description of the benthic foraminiferal record in outer Barilari Bay increases the understanding of the timing of events in the mid- to late Holocene and will serve as a correlation to other paleoclimate proxies from the LARISSA project.